1. Technical Field
The claimed invention relates to a terminal apparatus and a retransmission control method.
2. Description of the Related Art
3GPP LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme. In radio communication systems to which 3GPP LTE is applied, base stations transmit synchronization signals (i.e., Synchronization Channel: SCH) and broadcast signals (i.e., Broadcast Channel: BCH) using predetermined communication resources. Meanwhile, each terminal finds an SCH first and thereby ensures synchronization with a base station. Subsequently, the terminal reads BCH information to acquire base station-specific parameters (see, Non-Patent Literatures (hereinafter, abbreviated as NPL) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameters, each terminal sends a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via Physical Downlink Control CHannel (PDCCH) as appropriate to the terminal with which a communication link has been established.
The terminal performs “blind-determination” on each of a plurality of pieces of control information included in the received PDCCH signals (i.e., Downlink (DL) Assignment Control Information: also referred to as Downlink Control Information (DCI)). To put it more specifically, each piece of the control information includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part using the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the received piece of control information with its own terminal ID, the terminal cannot determine whether or not the piece of control information is intended for the terminal. In this blind-determination, if the result of demasking the CRC part indicates that the CRC operation is OK, the piece of control information is determined as being intended for the terminal.
Moreover, in 3GPP LTE, Automatic Repeat Request (ARQ) is applied to downlink data to terminals from a base station. To put it more specifically, each terminal feeds back response signals indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as response signals. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to feed back the response signals (i.e., ACK/NACK signals (hereinafter, may be referred to as “A/N,” simply)).
The control information to be transmitted from a base station herein includes resource assignment information including information on resources assigned to the terminal by the base station. As described above, PDCCH is used to transmit this control information. The PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH consists of one or more Control Channel Elements (CCE). To put it more specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH consists of a plurality of CCEs (2, 4 or 8), a plurality of contiguous CCEs starting from a CCE having an even index are assigned to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource assignment target terminal in accordance with the number of CCEs required for reporting the control information to the resource assignment target terminal. The base station maps the control information to physical resources corresponding to the CCEs of the L1/L2 CCH and transmits the mapped control information.
In addition, CCEs are associated with component resources of PUCCH (hereinafter, may be referred to as “PUCCH resource”) in a one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the component resources of PUCCH that correspond to the CCEs forming the L1/L2 CCH and transmits response signals to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the response signals to the base station using a PUCCH component resource corresponding to a CCE having a smallest index among the plurality of PUCCH component resources respectively corresponding to the plurality of CCEs (i.e., PUCCH component resource associated with a CCE having an even numbered CCE index). In this manner, the downlink communication resources are efficiently used.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristic of zero autocorrelation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in a PUCCH. In FIG. 1, (W0, W1, W2, W3) represent a length-4 Walsh sequence and (F0, F1, F2) represent a length-3 DFT sequence. As illustrated in FIG. 1, ACK or NACK response signals are primary-spread over frequency components corresponding to 1 SC-FDMA symbol by a ZAC sequence (length-12) in frequency-domain. To put it more specifically, the length-12 ZAC sequence is multiplied by a response signal component represented by a complex number. Subsequently, the ZAC sequence serving as the response signals and reference signals after the primary-spread is secondary-spread in association with each of a Walsh sequence (length-4: W0-W3 (may be referred to as Walsh Code Sequence)) and a DFT sequence (length-3: F0-F2). To put it more specifically, each component of the signals of length-12 (i.e., response signals after primary-spread or ZAC sequence serving as reference signals (i.e., Reference Signal Sequence) is multiplied by each component of an orthogonal code sequence (i.e., orthogonal sequence: Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into signals of length-12 in the time-domain by inverse fast Fourier transform (IFFT). A CP is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
The response signals from different terminals are spread using ZAC sequences each corresponding to a different cyclic shift value (i.e., index) or orthogonal code sequences each corresponding to a different sequence number (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, an orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed plurality of response signals using the related art despreading and correlation processing (see, NPL 4).
However, it is not necessarily true that each terminal succeeds in receiving downlink assignment control signals because the terminal performs blind-determination in each subframe to find downlink assignment control signals intended for the terminal. When the terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal would not even know whether or not there is downlink data intended for the terminal on the downlink component carrier. Accordingly, when a terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal generates no response signals for the downlink data on the downlink component carrier. This error case is defined as discontinuous transmission of ACK/NACK signals (DTX of response signals) in the sense that the terminal transmits no response signals.
In 3GPP LTE systems (may be referred to as “LTE system,” hereinafter), base stations assign resources to uplink data and downlink data, independently. For this reason, in the 3GPP LTE system, terminals (i.e., terminals compliant with LTE system (hereinafter, referred to as “LTE terminal”)) encounter a situation where the terminals need to transmit uplink data and response signals for downlink data simultaneously in the uplink. In this situation, the response signals and uplink data from the terminals are transmitted using time-division multiplexing (TDM). As described above, the single carrier properties of transmission waveforms of the terminals are maintained by the simultaneous transmission of response signals and uplink data using TDM.
In addition, as illustrated in FIG. 2, the response signals (i.e., “A/N”) transmitted from each terminal partially occupy the resources assigned to uplink data (i.e., Physical Uplink Shared CHannel (PUSCH) resources) (i.e., response signals occupy some SC-FDMA symbols adjacent to SC-FDMA symbols to which reference signals (RS) are mapped) and are thereby transmitted to a base station in time-division multiplexing (TDM). In FIG. 2, however, “subcarriers” in the vertical axis of the drawing are also termed as “virtual subcarriers” or “time contiguous signals,” and “time contiguous signals” that are collectively inputted to a discrete Fourier transform (DFT) circuit in a SC-FDMA transmitter are represented as “subcarriers” for convenience. To put it more specifically, optional data of the uplink data is punctured due to the response signals in the PUSCH resources. Accordingly, the quality of uplink data (e.g., coding gain) is significantly reduced due to the punctured bits of the coded uplink data. For this reason, base stations instruct the terminals to use a very low coding rate and/or to use very large transmission power so as to compensate for the reduced quality of the uplink data due to the puncturing.
Meanwhile, the standardization of 3GPP LTE-Advanced for realizing faster communications than 3GPP LTE has started. 3GPP LTE-Advanced systems (may be referred to as “LTE-A system,” hereinafter) follow 3GPP LTE systems (may be referred to as “LTE system,” hereinafter). 3GPP LTE-Advanced is expected to introduce base stations and terminals capable of communicating with each other using a wideband frequency of 40 MHz or greater to realize a downlink transmission rate up to 1 Gbps or above.
In the LTE-A system, in order to simultaneously achieve backward compatibility with the LTE system and ultra-high-speed communications several times faster than transmission rates in the LTE system, the LTE-A system band is divided into “component carriers” of 20 MHz or below, which is the bandwidth supported by the LTE system. In other words, the “component carrier” is defined herein as a band having a maximum width of 20 MHz and as the basic unit of communication band. Moreover, “component carrier” in downlink (hereinafter, referred to as “downlink component carrier”) is defined as a band obtained by dividing a band according to downlink frequency bandwidth information in a BCH broadcasted from a base station or as a band defined by a distribution width when a downlink control channel (PDCCH) is distributed in the frequency domain. In addition, “component carrier” in uplink (hereinafter, referred to as “uplink component carrier”) may be defined as a band obtained by dividing a band according to uplink frequency band information in a BCH broadcasted from a base station or as the basic unit of a communication band of 20 MHz or below including a Physical Uplink Shared CHannel (PUSCH) in the vicinity of the center of the bandwidth and PUCCHs for LTE on both ends of the band. In addition, the term “component carrier” may be also referred to as “cell” in English in 3GPP LTE-Advanced.
The LTE-A system supports communications using a band obtained by aggregating several component carriers, so called “carrier aggregation.” In general, throughput requirements for uplink are different from throughput requirements for downlink. For this reason, so called “asymmetric carrier aggregation” has been also discussed in the LTE-A system. In asymmetric carrier aggregation, the number of component carriers configured for any terminal compliant with the LTE-A system (hereinafter, referred to as “LTE-A terminal”) differs between uplink and downlink. In addition, the LTE-A system supports a configuration in which the numbers of component carriers are asymmetric between uplink and downlink, and the component carriers have different frequency bandwidths.
FIG. 3 is a diagram provided for describing asymmetric carrier aggregation and a control sequence applied to individual terminals. FIG. 3 illustrates a case where the bandwidths and numbers of component carriers are symmetric between the uplink and downlink of base stations.
As illustrated in FIG. 3B, a configuration in which carrier aggregation is performed using two downlink component carriers and one uplink component carrier on the left is set for terminal 1, while a configuration in which the two downlink component carriers identical with those used by terminal 1 are used but uplink component carrier on the right is used for uplink communications is set for terminal 2.
Referring to terminal 1, an LTE-A base station and an LTE-A terminal included in the LTE-A system transmit and receive signals to and from each other in accordance with the sequence diagram illustrated in FIG. 3A. As illustrated in FIG. 3A, (1) terminal 1 is synchronized with the downlink component carrier on the left when starting communications with the base station and reads information on the uplink component carrier paired with the downlink component carrier on the left from a broadcast signal called system information block type 2 (SIB2). (2) Using this uplink component carrier, terminal 1 starts communications with the base station by transmitting, for example, a connection request to the base station. (3) Upon determining that a plurality of downlink component carriers need to be assigned to the terminal, the base station instructs the terminal to add a downlink component carrier. However, in this case, the number of uplink component carriers is not increased, and terminal 1, which is an individual terminal, starts asymmetric carrier aggregation.
In addition, in the LTE-A system to which carrier aggregation is applied, a terminal may receive a plurality of pieces of downlink data on a plurality of downlink component carriers at a time. In LTE-A, studies have been carried out on channel selection (also referred to as “multiplexing”), bundling and a discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) format as a method of transmitting a plurality of response signals for the plurality of pieces of downlink data. In channel selection, not only symbol points used for response signals, but also the resources to which the response signals are mapped are varied in accordance with the pattern for results of the error detection on the plurality of pieces of downlink data. Compared with channel selection, in bundling, ACK or NACK signals generated according to the results of error detection on the plurality of pieces of downlink data are bundled (i.e., bundled by calculating a logical AND of the results of error detection on the plurality of pieces of downlink data, provided that ACK=1 and NACK=0), and response signals are transmitted using one predetermine resource. In transmission using the DFT-S-OFDM format, a terminal jointly encodes (i.e., joint coding) the response signals for the plurality of pieces of downlink data and transmits the coded data using the format (see, NPL 5). For example, a terminal may feed back the response signals (i.e., ACK/NACK) using channel selection, bundling or DFT-S-OFDM according to the number of bits for a pattern for results of error detection. Alternatively, a base station may previously configure the method of transmitting the response signals.
More specifically, channel selection is a technique that varies not only the phase points (i.e., constellation points) for the response signals but also the resources used for transmission of the response signals (may be referred to as “PUCCH resource,” hereinafter) on the basis of whether the results of error detection on the plurality of pieces of downlink data received on the plurality of downlink component carriers are each an ACK or NACK as illustrated in FIG. 4. Meanwhile, bundling is a technique that bundles ACK/NACK signals for the plurality of pieces of downlink data into a single set of signals and thereby transmits the bundled signals using one predetermined resource (see, NPLs 6 and 7). Hereinafter, the set of the signals formed by bundling ACK/NACK signals for a plurality of pieces of downlink data into a single set of signals may be referred to as “bundled ACK/NACK signals.”
The following two methods are considered as a possible method of transmitting response signals in uplink when a terminal receives downlink assignment control information via a PDCCH and receives downlink data.
One of the methods is to transmit response signals using a PUCCH resource associated in a one-to-one correspondence with a control channel element (CCE) occupied by the PDCCH (i.e., implicit signaling) (hereinafter, method 1). More specifically, when DCI intended for a terminal served by a base station is allocated in a PDCCH region, each PDCCH occupies a resource consisting of one or a plurality of contiguous CCEs. In addition, as the number of CCEs occupied by a PDCCH (i.e., the number of aggregated CCEs: CCE aggregation level), one of aggregation levels 1, 2, 4 and 8 is selected according to the number of information bits of the assignment control information or a propagation path condition of the terminal, for example.
The other method is to previously report a PUCCH resource to each terminal from a base station (i.e., explicit signaling) (hereinafter, method 2). To put it differently, each terminal transmits response signals using the PUCCH resource previously reported by the base station in method 2.
In addition, as illustrated in FIG. 4, one of the two downlink component carriers is paired with one uplink component carrier to be used for transmission of response signals. The downlink component carrier paired with the uplink component carrier to be used for transmission of response signals is called a primary component carrier (PCC) or a primary cell (PCell). In addition, the downlink component carrier other than the primary component carrier is called a secondary component carrier (SCC) or a secondary cell (SCell). For example, PCC (or PCell) is the downlink component carrier used to transmit broadcast information about the uplink component carrier on which response signals to be transmitted (e.g., system information block type 2 (SIB 2)).
In method 2, PUCCH resources common to a plurality of terminals (e.g., four PUCCH resources) may be previously reported to the terminals from a base station. For example, terminals may employ a method to select one PUCCH resource to be actually used, on the basis of a transmit power control (TPC) command of two bits included in DCI in SCell. In this case, the TPC command is called an ACK/NACK resource indicator (ARI). Such a TPC command allows a certain terminal to use an explicitly signaled PUCCH resource in a certain frame while allowing another terminal to use the same explicitly signaled PUCCH resource in another subframe in the case of explicit signaling.
Meanwhile, in channel selection, a PUCCH resource in an uplink component carrier associated in a one-to-one correspondence with the top CCE index of the CCEs occupied by the PDCCH indicating the PDSCH in PCC (PCell) (i.e., PUCCH resource in PUCCH region 1 in FIG. 4) is assigned (implicit signaling).
Next, a description will be provided regarding ARQ control using channel selection when the asymmetric carrier aggregation described above is applied to terminals with reference to FIGS. 4 and 5.
In a case where a component carrier group (may be referred to as “component carrier set” in English) consisting of downlink component carrier 1 (PCell), downlink component carrier 2 (SCell) and uplink component carrier 1 is configured for terminal 1 as illustrated in FIG. 4, after downlink resource assignment information is transmitted via a PDCCH of each of downlink component carriers 1 and 2, downlink data is transmitted using the resource corresponding to the downlink resource assignment information.
In channel selection, when terminal 1 succeeds in receiving the downlink data on component carrier 1 (PCell) but fails to receive the downlink data on component carrier 2 (SCell) (i.e., when the result of error detection on component carrier 1 (PCell) is an ACK and the result of error detection on component carrier 2 (SCell) is a NACK), the response signals are mapped to a PUCCH resource in PUCCH region 1 to be implicitly signaled, while a first phase point (e.g., phase point (1, 0) and/or the like) is used as the phase point of the response signals. In addition, when terminal 1 succeeds in receiving the downlink data on component carrier 1 (PCell) and also succeeds in receiving the downlink data on component carrier 2 (SCell), the response signals are mapped to a PUCCH resource in PUCCH region 2 while the first phase point is used. More specifically, when the number of downlink component carriers is two while there is a single codeword (CW) per downlink component carrier, the results of error detection are represented in four patterns (i.e., ACK/ACK, ACK/NACK, NACK/ACK and NACK/NACK). The four patterns can be represented by combinations of two PUCCH resources and two kinds of phase points (e.g., binary phase shift keying (BPSK) mapping).
In addition, when terminal 1 fails to receive DCI on component carrier 1 (PCell) but succeeds in receiving downlink data on component carrier 2 (SCell) (i.e., the result of error detection on component carrier 1 (PCell) is a DTX and the result of error detection on component carrier 2 (SCell) is an ACK), the CCEs occupied by the PDCCH intended for terminal 1 cannot be identified. Thus, the PUCCH resource included in PUCCH region 1 and associated in a one-to-one correspondence with the top CCE index of the CCEs cannot be identified either. Accordingly, in this case, in order to report an ACK, which is the result of error detection on component carrier 2, the response signals need to be mapped to an explicitly signaled PUCCH resource included in PUCCH region 2 (may be referred to as “to support implicit signaling,” hereinafter).
To be more specific, FIG. 5 illustrates examples of mapping of patterns for the results of error detection in the following cases: when there are two downlink component carriers (one PCell and one SCell), and
(a) Single CW per downlink component carrier;
(b) Single CW for one of the downlink component carriers, and two CWs for the other; and
(c) Two CWs per downlink component carrier. The number of patterns for results of error detection for (a) is four (i.e., 22=4). The number of patterns for (b) is eight (i.e., 23=8). The number of patterns for (c) is 16 (i.e., 24=16). The number of PUCCH resources required for mapping all the patterns is at least one for (a), at least two for (b) and at least four for (c) when the phase difference between phase points is a minimum of 90 degrees (i.e., when a maximum of four patterns per PUCCH resource is mapped).
In FIG. 5A, one PUCCH resource is sufficient when mapping is performed using QPSK because there are only four patterns for results of error detection. However, in order to improve the degree of freedom in mapping and the error rate in reporting response signals to the base station, BPSK mapping may be carried out using two PUCCH resources as illustrated in FIG. 5A. In the mapping illustrated in FIG. 5A, the base station can determine the result of error detection on component carrier 2 (SCell) only by determining in which one of the PUCCH resources the response signals are reported.
Meanwhile, the base station cannot determine the result of error detection on component carrier 1 (PCell) only by determining in which one of the PUCCH resources the response signals are reported. The base station can determine whether the result of error detection is an ACK or NACK further by determining to which pattern on BPSK the response signals are mapped.
As described, the method used by the base station to determine response signals varies depending on the mapping method. As a result, the error rate characteristics vary for each set of response signals. To put it differently, determining the ACK or NACK by only determining in which one of the PUCCH resources the response signals are reported (hereinafter, may be referred to as “determination method 1”) has fewer errors than determining the ACK or NACK by determining in which one of the PUCCH resources the response signals are reported and further determining the phase point of the PUCCH resource (hereinafter, may be referred to as “determination method 2”).
Likewise, in FIG. 5B, the error rate characteristics of the set of response signals for CW0 of component carrier 1 (PCell) indicate fewer errors than the error rate characteristics of the other two sets of response signals. In FIG. 5C, the error rate characteristics of the response signals for two CWs (CW0, CW1) of component carrier 1 (PCell) indicate fewer errors than the error rate characteristics of the response signals for two CWs (CW0, CW1) of component carrier 2 (SCell).
Meanwhile, there is a period in which the understanding about the number of CCs configured for a terminal is different between a base station and the terminal (i.e., uncertainty period or misalignment period). The base station notifies the terminal of a message indicating reconfiguration to change the number of CCs, and upon reception of the message, the terminal understands that the number of CCs has been changed and notifies the base station of a completion message for the reconfiguration of the number of CCs. The period in which the understanding about the number of CCs configured for a terminal is different between a base station and the terminal stems from the fact that the base station understands, upon reception of the message, for the first time, that the number of CCs configured for the terminal has been changed.
For example, when the terminal understands that the number of CCs configured for the terminal is one while the base station understands that the number of CCs configured for the terminal is two, the terminal transmits response signals for the data that has been received by the terminal, using the mapping pattern for the result of error detection corresponding to one CC. Meanwhile, the base station determines the response signals from the terminal for the data that has been transmitted to the terminal, using the mapping pattern for the results of error detection corresponding to two CCs.
When the number of CCs is one, the mapping pattern for a result of error detection for one CC that is used in the LTE system is used (may be referred to as “LTE fallback,” hereinafter) in order to ensure backward compatibility with the LTE system. More specifically, when one CC performs single-CW processing, an ACK is mapped to the phase point (−1, 0) and a NACK is mapped to the phase point (1, 0) using BPSK mapping (may be referred to as “fallback to Format 1a,” hereinafter) as illustrated in FIG. 6A. As illustrated in 6B, when one CC performs two-CW processing, ACK/ACK, ACK/NACK, NACK/ACK and NACK/NACK are mapped to the phase points (−1, 0), (0, 1), (0, −1), and (1, 0), respectively, using QPSK mapping (may be referred to as “fallback to Format 1b,” hereinafter).
To be more specific, a description will be provided using an example of a case where the base station transmits one piece of single-CW data on PCell and one piece of single-CW data on SCell using the two CCs when the terminal understands that the number of CCs configured for the terminal is one while the base station understands that the number of CCs configured for the terminal is two. Since the terminal understands that the number of CCs configured for the terminal is one, the terminal receives only PCell. When succeeding in receiving the downlink data in PCell, the terminal maps the response signals using the mapping illustrated in FIG. 6A to the PUCCH resource in the uplink component carrier (PUCCH resource 1) associated in a one-to-one correspondence with the top CCE index of the CCEs occupied by the PDCCH indicating the PDSCH in PCell (i.e., implicitly signaled). In short, the terminal uses the phase point (−1, 0). Meanwhile, the base station determines the response signals using the mapping illustrated in FIG. 5A since the base station understands that the number of CCs configured for the terminal is two. In other words, the base station can determine that single CW of PCell is an ACK and single CW of SCell is a NACK or DTX because of the phase point (−1, 0) of PUCCH resource 1. Likewise, when failing to receive the downlink data in PCell, the terminal needs to map the response signals to the phase point (1, 0).
The same applies to the case where the way the understanding about the number of CCs is different between the base station and the terminal is opposite to the case described above. To put it more specifically, this case is where the base station transmits one piece of single-CW data on PCell to the terminal using the one CC when the terminal understands that the number of CCs configured for the terminal is two while the base station understands that the number of CCs configured for the terminal is one. Since the terminal understands that the number of CCs configured for the terminal is two, the terminal receives PCell and SCell. When the terminal succeeds in receiving the downlink data in PCell, the base station expects to receive, using the mapping illustrated in FIG. 6A, the response signals mapped to the phase point (−1, 0) of the PUCCH resource in the uplink component carrier (PUCCH resource 1) associated in a one-to-one correspondence with the top CCE index of the CCEs occupied by the PDCCH indicating the PDSCH in PCell (implicitly signaled). Accordingly, although the terminal understands that the number of CCs is two, the terminal needs to map the response signals to the phase point (−1, 0) of PUCCH resource 1 as illustrated in FIG. 5A when single CW of PCell is an ACK and SCell is a DTX. Likewise, when failing to receive the downlink data in PCell, the terminal needs to map the response signals to the phase point (1, 0).
As described above, even when the understanding about the number of CCs configured for a terminal is different between a base station and the terminal, the response signals on PCell and SCell need to be correctly determined (may be referred to as “to support LTE fallback,” hereinafter).
FIG. 5A supports LTE fallback. More specifically, FIG. 5A supports LTE fallback to PUCCH format 1a. FIG. 5B does not support LTE fallback because A/A/D is not mapped to the phase point (−1, 0) of PUCCH resource 1 when PCell performs two-CW processing and SCell performs single-CW processing. More specifically, FIG. 5B does not support LTE fallback to PUCCH format 1a. In addition, FIG. 5B does not support LTE fallback because A/D/D is not mapped to the phase point (−1, 0) of PUCCH resource 1, A/N/D is not mapped to the phase point (0, 1) of PUCCH resource 1, and N/A/D is not mapped to the phase point (0, −1) either when PCell performs single-CW processing and SCell performs two-CW processing. More specifically, FIG. 5B does not support LTE fallback to PUCCH format 1b. FIG. 5C does not support LTE fallback because A/A/D/D is not mapped to the phase point (−1, 0) of PUCCH resource 1, A/N/D/D is not mapped to the phase point (0, 1) of PUCCH resource 1, and N/A/D/D is not mapped to the phase point (0, −1) of PUCCH resource 1 either. More specifically, FIG. 5C does not support LTE fallback to PUCCH format 1b. 
In the mapping method disclosed in Non-Patent Literature (hereinafter, abbreviated as NPL) 8 (may be referred to as “transmission rule table” or “mapping table”) (FIGS. 7 and 8), two ACK/NACK bits (may be referred to as “HARQ-ACK” bit) (correspond to b0 and b1 in NPL 9) in case of “four ACK/NACK Bits” in FIG. 8, for example, can be always determined by determination method 1. However, the remaining two ACK/NACK bits (corresponding to b2 and b3 in NPL 9) in the “four ACK/NACK Bits” in FIG. 8 are always determined by determination method 2. An evaluation result using the abovementioned mapping is disclosed in NPL 9, and it can be seen that NACK-to-ACK characteristics of b2 and b3 are poor as compared with b0 and b1.
In the mapping method disclosed in NPL 10 (FIG. 9), the number of PUCCH resources that can be determined by determination method 1 is smoothed out among the bits. More specifically, it is possible to determine b3 in PUCCH 1, b0 and b1 in PUCCH 2, b1 and b2 in PUCCH 3, and b3 in PUCCH 4 by determination method 1. In FIG. 9, the number of PUCCH resources that can be determined by determination method 1 for each bit is one with b0, two with b1, one with b2 and two with b3. Furthermore, NPL 10 discloses nothing about associations between PUCCH 1 and b0, PUCCH 2 and b1, PUCCH 3 and b2, and PUCCH 4 with b3, but if they are associated with each other, implicit signaling for an optional ACK/NACK bit is supported in NPL 10. However, this mapping cannot support LTE fallback in two CCs.